1. Field of the Invention
The present invention relates to an optical communication module and a flexible printed circuit board. In particular, the present invention relates to a structure of a flexible printed circuit board for connecting a main board of an optical communication module having a high speed transmission rate (e.g., 10 Gbit/s) with a small optical device for transmission or reception.
2. Description of the Related Art
In recent years, downsizing and speeding up of a transceiver module for optical fiber transmission (optical transceiver module) has been achieved along with the widespread use of broadband networks. Concerning the speeding up, optical transceiver modules having a bit rate of 10 Gbit/s are widely used at present. As to the downsizing, a case volume has been reduced from the 300 pin multi source agreement (MSA) standard of the old generation to XENPAK (10 gigabit Ethernet transceiver package), X2, XFP (10 gigabit small form factor pluggable), and SFP+ (small form factor pluggable) (MSA standard each).
Such an optical transceiver module uses a transmitter optical sub-assembly (TOSA) having an optical connector interface and a receiver optical sub-assembly (ROSA) having an optical connector interface, and hence downsizing of the module is realized. In addition, most of the optical transceiver modules use a flexible printed circuit board (FPC) having flexibility for connecting the TOSA or the ROSA with the main board, and hence a stress due to a dimensional error in the mounting process or a thermal distortion is absorbed for preventing a failure caused by the stress (see, for example, “850 nm XFP Transceiver Reference Design HFRD-19.2 (Rev. 2, October 2007)”, (online), Maxim, retrieved on Dec. 24, 2007, the Internet URL: http://pdfserv.maxim-ic.com/jp/an/AN3730J.pdf (hereinafter referred to as “Non-Patent Document 1”).
Further, JP 2004-193489 A (hereinafter referred to as “Patent Document 1”) discloses an optical transmission module in which a TOSA is driven by a differential driver IC. In addition, Patent Document 1 discloses in Paragraph 0020 and in FIG. 5 a technology for improving waveform deterioration of an optical signal generated in the TOSA by disposing a resistor element between an end of a semiconductor laser device and the ground potential so as to improve impedance mismatch between a differential transmission line and the semiconductor laser device.
In addition, JP 3084970 B2 (hereinafter referred to as “Patent Document 2”) discloses a technology for improving impedance mismatch by inserting resistor elements in series in a signal wiring on an FPC.
In a 10 Gbit/s optical transceiver module having a transmission distance of 300 meters or shorter, a vertical cavity surface emitting laser (VCSEL) device having a wavelength of 0.85 μm is widely used as the semiconductor laser device for transmission. This device has features that a series resistance is relatively high like 50 to 100 ohms and that necessary drive current is small like several milliamperes.
According to the study carried out by the inventors of the present invention, if an output signal (differential signal) of a balanced (differential) driver IC having a differential output impedance of 100 ohms is supplied to an anode terminal and a cathode terminal of the semiconductor laser device via a balanced (differential) transmission line (differential line) having characteristic impedance of 100 ohms, impedance matching between the transmission line and the laser device can be performed well, and hence an optical output waveform of the optical transceiver module can be improved.
On the other hand, as to a common mode operation, it is possible to maintain the matching between the balanced driver IC and the differential line. However, the semiconductor laser device connected in series to the ends of the two lines is in the open state with respect to the common mode operation, and hence a common mode signal is totally reflected. Therefore, depending on characteristics of the driver IC, if the common mode signal that is a parasitic component that is not originally essential for the output of the driver IC is superimposed on the output signal as noise, a large electromagnetic radiation may be generated by the impedance mismatch, which causes a problem that the specification about unintentional electromagnetic radiation cannot be satisfied. Note that generation factors of the common mode signal include a switching operation of the driver IC or the cross point adjustment.
Concerning this point, the technology described in the above-mentioned Patent Document 1 can avoid the impedance mismatch with respect to the common mode signal by setting a resistance value of a resistor element for impedance matching disposed between the terminal of the laser device and the ground potential to be low (e.g., 50 ohms or its vicinity). In this case, however, high frequency current of the differential signal for driving the laser device and bias current of the laser device are consumed by the resistor element for the impedance matching. Therefore, there is a problem that higher drive current and bias current are necessary for obtaining a desired good output optical waveform, which causes an increase in power consumption.
On the other hand, if the technology disclosed in Patent Document 2 is applied to the optical transceiver module disclosed in Non-Patent Document 1, the impedance mismatch with respect to the common mode signal cannot be avoided. Specifically, if resistor elements are inserted in series in a signal wiring on the FPC of the optical transceiver module disclosed in Non-Patent Document 1, the semiconductor laser device is in the open state with respect to the common mode operation. Therefore, the impedance mismatch cannot be resolved even if the resistor elements are inserted in series in the signal wiring on the FPC. In this case, therefore, the problem of the unintentional electromagnetic radiation cannot be solved.